1. Technical Field of the Invention
This invention relates to a bulkhead assembly for use with double walled metal pipes, or “pipe-in-pipe” systems, for use in submarine pipelines.
2. Relevant Art
It is known to use double walled or “pipe-in-pipe” tubular members as submarine pipelines. An inner tube forms the actual flowline, while an outer tube defines an annular space between itself and the inner tube. The annular space is typically filled with an insulating material to reduce heat loss from fluid flowing in the pipeline.
It is usual to provide bulkheads in the form of annular members interconnecting the inner and outer tubes. These bulkhead are designed for transferring mechanical loads between the tubes during installation and operation. These loads can be laying loads (handling loads which have to be transferred to the inner pipe, for example) or service loads due to the effect of temperature of the transported liquid on the pipeline. Bulkheads are typically located at spacing of about 1 km, but these spacings can be reduced or increased according to the conditions of the particular application.
The general prior art in pipe-in-pipe systems provides bulkheads by means of a variety of forged or welded assemblies. Such prior art, and its disadvantages, are discussed in our published International Patent Application No WO 01/55630.
FIG. 1 shows another form of a bulkhead assembly devised by the present Applicant and forms one embodiment of the above International patent application. In FIG. 1, a double-walled rigid pipe 1 having a longitudinal axis A comprises an inner wall or tube 2 (the “flow pipe”), the diameter 16 at a thinner portion 14 of tube 2 and the material of which are chosen according to the nature, pressure and temperature of the fluid 5 to be carried, and an outer wall or tube (the “carrier pipe”) with a diameter D which is slipped over the inner tube 2. A bulkhead assembly 10 essentially comprises an annular partition 11 having a width E fastened to the tubes 2 and 3. In the example shown, the annular partition 11 is in screw-threaded engagement with a thread 13 formed on a thickened portion 15 of the inner tube 2.
The arrangement shown in FIG. 1 shows an excellent mechanical performance, and is easy to produce and assemble. However, when pipeline is being laid a differential torque strain can be produced between the inner and outer tubes because of the difference between the trajectory of the laying vessel and the subsea path of the pipeline, due to environmental reasons such as waves and currents. Torque strains can also be induced by anisotropy of the material, or by the spooling on and off operations. This can produce the effect of unscrewing the threaded connection, which has two possible adverse consequences.
One is that the loads which the bulkhead transfers between the inner and outer tubes are transferred through the threaded portion, which suitably is of trapezoidal thread ACME type. Unscrewing of this portion can cause the number of threads that are engaged to be reduced and thus the maximum load to be reduced.
The other is that in cases where the bulkhead is also used to provide fluid containment, for example to provide a water containment when the external pipe is damaged, at least one end of the threaded portion is arranged to provide sealing, e.g. by way of welding or the inclusion of a sealing element. The unscrewing of the bulkhead can damage the integrity of the seal.